1. Field of the Invention
The present invention relates to an apparatus and a method for producing polycrystalline silicon having a reduced amount of boron compounds. Especially, for reducing the amount boron compounds, an unpurified trichlorosilane is mixed with a condensate of a regeneration gas discharged from an activated carbon tower.
2. Description of Related Art
Polycrystalline silicon is produced by the reaction of trichlorosilane and hydrogen gas, in a process known as the Siemens method. In the Siemens method, high-purity polycrystalline silicon deposits on polycrystalline silicon rods by hydrogen reduction of trichlorosilane shown by formula (1) and thermal decomposition of trichlorosilane shown by formula (2):SiHCl3+H2→Si+3HCl  (1)4SiHCl3→Si+3SiCl4+2H2  (2).
Trichlorosilane (SiHCl3, abbreviated “TCS”, boiling point: 31.8° C.), used as a raw material for producing high purity polycrystalline silicon, is produced by reacting metallurgical grade silicon powder (abbreviated “Me—Si”) of about 98% purity, which includes boron impurities, with hydrogen chloride gas (abbreviated “HCl”). Because other reactants are also produced in the reaction, a distillation process follows the reaction of TCS and HCl.
Trichlorosilane is purified by the distilling process. However, it is very difficult to separate trichlorosilane and boron compounds, produced in the reaction, which have low boiling points like diborane (B2H6) (boiling point: −92.5° C.), boron trichloride (BCl3) (boiling point: 12.4° C.), tetraborane (B4H10) (boiling point: 18° C.), etc., by commercial distillation processes, because the boiling point of many boron compounds are close to or lower than that of TCS. Boron is included in metallurgical grade silicon powder as an unavoidable impurity. Several different boron compounds are created in the TCS and HCl reaction.
Some methods for producing trichlorosilane are proposed for removing boron compounds, for example as disclosed in Japanese Unexamined Patent Application Publication No. 2005-67979. The application proposes a method in which an ether group is added to an unpurified chlorosilane, then the unpurified chlorosilane is distilled. However, ether group recovery followed by refining is necessary. Further, U.S. Pat. No. 4,713,230 proposes a process for purification of trichlorosilane in which the vapor phase trichlorosilane, contaminated with boron compounds, is passed through a bed of silica. But a fixed bed of silica is required to maintain the cleaning of the silica.
One object of this present invention is to provide an apparatus and a method for manufacturing polycrystalline silicon having a reduced amount of boron compounds.